The present invention relates to a microwave cooking oven and specifically to a rack for use in the oven whereby two levels of food may be cooked simultaneously.
In a microwave oven cooking cavity, the spatial distribution of the microwave energy tends to be non-uniform. As a result, "hot spots" and "cold spots" are produced at different locations. For many types of foods, cooking results are unsatisfactory under such conditions because some portions of the food may be completely cooked while others are barely warmed. The problem becomes more severe with foods of low thermal conductivity which do not readily conduct heat from the areas which are heated by the microwave energy to those areas which are not. An example of a food falling within this class is cake. However, other foods frequently cooked in microwave ovens, such as meat, also produce unsatisfactory cooking results if the distribution of microwave energy within the oven cavity is not uniform.
One explanation for the non-uniform cooking pattern is that electromagnetic standing wave patterns, known as "modes," are set up within the cooking cavity. When a standing wave pattern is established, the intensities of the electric and magnetic fields vary greatly with position. The precise configuration of the standing wave or mode pattern is dependent at least upon the frequency of microwave energy used to excite the cavity and upon the dimensions of the cavity itself. It is possible to theoretically predict the particular mode patterns which may be present in the cavity, but actual experimental results are not always consistent with theory. This is particularly so in a countertop microwave oven operating at a frequency of 2450 MHz. Due to the relatively large number of theoretically possible modes within a given rectangular cavity, it is difficult to predict with certainty which of the modes will dominate. The situation is further complicated by the differing loading effects of different types and quantities of food which may be placed in the cooking cavity.
A number of different approaches to altering the standing wave patterns have been tried to an effort to alleviate the problem of non-uniform energy distribution. The most common approach is the use of a device known as a "mode stirrer," which typically resembles a fan having metal blades. The mode stirrer rotates and may be placed either within the cooking cavity itself (usually protected by a cover constructed of a material transparent to microwaves) or, to conserve space within the cooking cavity, it may be mounted within a recess formed in one of the cooking cavity walls, normally the top.
The function of the mode stirrer is to continually alter the mode pattern in the oven cavity. As a result of continually changing the mode pattern in the cavity, the "hot" and "cold" spots are continually shifted and, when averaged over a period of time, the energy distribution in the cavity is made more uniform.
The proper distribution of microwave energy, however, is particularly difficult where it is desired to cook two levels of food simultaneously. To do so a rack is needed to place one level of food above the other. These racks are normally made of plastic, however, it is desirable that the rack is made of metal. Normally metal racks used in a microwave oven detrimentally affects the distribution of the microwave energy. It is particularly difficult to arrange for two levels of food to be cooked with the same amount of microwave energy available to both levels. It is desirable to have such equal distribution of microwave energy as otherwise, one level would cook faster than the other and this, of course, is unsatisfactory.
By this invention, there is provided a rack for use in a microwave oven cavity which may be made of metal and does not detrimentally affect the microwave energy pattern in the cavity and does allow for the same amount of microwave energy available to both levels of food.